1. Field of the Invention
The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus having an intermediate gear bearing that rotatably supports an intermediate gear for transmitting drive force.
2. Background Information
In conventional practice, structures having intermediate gear bearings that rotatably support intermediate gears are known.
Also, heat transfer printers are conventionally known as an example of image forming apparatuses. The structure of the heat transfer printer according to a conventional example will now be described with reference to FIGS. 13 through 16.
A heat transfer printer is generally equipped with a motor for driving various rollers. Such motor generally has a motor gear that engages a drive gear, such as a drive gear 113 shown in FIG. 14. The motor transmits the torque to a feed roller 115 via the drive gear 113 and an intermediate gear 114. The drive gear 113 is provided with an axle 113c, a large gear 113a that meshes with the motor gear, and a small gear 113b that meshes with a large gear 114a provided to the intermediate gear 114 and has a smaller diameter than the large gear 113a. Also, the intermediate gear 114 is provided with an axle 114c, the large gear 114a that meshes with the small gear 113b of the drive gear 113, a small gear 114b that meshes with the feed roller gear 115 and has a smaller diameter than the large gear 114a. The axle 114c of the intermediate gear 114 is rotatably supported by an intermediate gear bearing 109b and an intermediate gear bearing 110b. 
As shown in FIG. 13, the intermediate gear bearing 109b and the intermediate gear bearing 110b have circular inner surfaces. As shown in FIGS. 15 and 16, the intermediate gear bearing 110b and the intermediate gear bearing 109b support both ends of the axle 114c at contact positions Q.
As shown in FIG. 14, when the intermediate gear 114 receives the force P6 as the resultant force of the force P4 the large gear 114a receives from the small gear 113b of the drive gear 113 and the force P5 received as a reaction when the small gear 114b rotates the feed roller gear 115. The intermediate gear bearing 10b shown in FIG. 15 and the intermediate gear bearing 109b shown in FIG. 16 are thereby pressed by the axle 114c of the intermediate gear 114 along the line of the resultant force P6.
As the axle 114c of the intermediate gear 114 rotates in the direction of arrow F2, frictional force μP6 (μ is a dynamic friction coefficient) acts on the contact part Q of the axle 114c. As a result of this frictional force μP6, the axle 114c of the intermediate gear 114 begins to move along the inner surfaces of the intermediate gear bearing 110b and the intermediate gear bearing 109b. The axle 114c of the intermediate gear 114 then stops moving at a position inclined by an angle θ3, at which the following equation is satisfied:μP6 cos θ3 (the frictional force)=P6 sin θ3 (the resultant force)The axle 114c of the intermediate gear 114 is thereby rotated while maintaining a contact with the intermediate gear bearings 109b and 110b at the contact part Q, which is inclined by the angle θ3.
In the conventional heat transfer printer shown in FIGS. 13 through 16, the frictional force μP6 increases or decreases when the resultant force P6 received by the axle 114c of the intermediate gear 114 varies as a result of variations in the force received by the paper during the printing operation. This is disadvantageous in that the axle 114c of the intermediate gear 114 moves along the inner surfaces of the intermediate gear bearing 110b and the intermediate gear bearing 109b. 
Specifically, when the load received by the paper during the printing operation increases and the force P5 (see FIG. 14) for rotating the feed roller 102 that conveys the paper becomes greater, the frictional force μP6 (see FIGS. 15 and 16) that is generated as the axle 114c of the intermediate gear 114 rotates also increases. Accordingly, the contact point at which the axle 114c of the intermediate gear 114 contacts the intermediate gear bearings 109b and 110b moves so that the angle θ3 widens.
Conversely, when the load received by the paper during the printing operation decreases and the reaction P5 (see FIG. 14) for rotating the feed roller 102 that conveys the paper becomes smaller, the frictional force μP6 (see FIGS. 15 and 16) for moving the axle 114c of the intermediate gear 114 decreases. Accordingly, the contact point at which the axle 114c of the intermediate gear 114 contacts the intermediate gear bearings 109b and 110b moves so that the angle θ3 is narrowed. At this time, the axle 114c of the intermediate gear 114 moves along the inner surfaces of the intermediate gear bearing 110b and the intermediate gear bearing 109b without rotating the intermediate gear 114, which is disadvantageous in that the amount of rotation of the intermediate gear 114 becomes inconsistent.
In conventional structure, as described above, when the amount of rotation of the intermediate gear 114 varies, the rotation of the feed roller gear 115 also varies, causing undesirable variations in the distance by which the paper is conveyed by the feed roller 102. In this case, paper is fed non-uniformly. Thus, it is difficult to precisely control the conveyance of paper with the feed roller 102.
Furthermore, in conventional structure, in order to reduce such paper feeding non-uniformities, the intermediate gear 114 and the intermediate gear bearings 109b and 110b are formed so that there is a relatively tight fit between the outer surface of the axle 114c of the intermediate gear 114 and the inner surfaces of the intermediate gear bearing 10b and the intermediate gear bearing 109b. Therefore, high precision is required of the intermediate gear 114 and the structures in which the intermediate gear bearings 109b and 110b are formed. Thus, component costs are high.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved image forming apparatus that overcomes the problems of the conventional art. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.